The field of the present invention relates to optical systems for data reading and particularly to a scanning system having improved resistance to shock and vibration.
Typically, a data reading device such as a bar code scanner illuminates a bar code and senses light reflected from the code to detect the bars and spaces of the code symbols and thereby derive the encoded data. In a common system, the scanner includes a housing and a scan module comprising a light source, one or more scanning mechanisms, a detector, and optics and signal processing circuitry.
A variety of scan modules and their scanning mechanisms are known as described in, for example, U.S. Pat. Nos. 5,475,206 and 5,629,510 or U.S. application Ser. No. 08/934,487, each of these disclosures hereby incorporated by reference. Such scanning mechanisms typically comprise rotating polygon mirror assemblies and dithering or oscillating mirror assemblies. Dithering assemblies typically comprise a cantilevered mirror and a drive means or dithering motor for moving the mirror.
FIG. 1 illustrates a dithering assembly 100 comprising a mirror/magnet assembly 110, drive coil 106, feedback coil 108, bending member or flexure 112, and mounting member 114. The mounting member 114 is mounted to a suitable chassis (not shown). The mirror/magnet assembly comprises mirror 102, mirror bracket 103, drive magnet 104, and feedback magnet 105. The bracket 103 holds mirror 102 and is pivotally supported on the mounting member 114 via flexure 112. Bending of flexure 112 results in rotation of the mirror/magnet assembly 110 about an axis substantially parallel to mirror 102, perpendicular to the plane of FIG. 1.
Due to the cantilevered ditherer configuration and the sensitive components used to construct the scanner, current scanners are relatively sensitive to shock and are often damaged before they would have worn out for other reasons. Handheld scanners are particularly subjected to shock and have been equipped with shock protection such as by mounting the scan module to the interior of the housing body via shock mounts as described in U.S. Pat. No. 5,475,206.
Other shock protection includes a pin-in-hole arrangement that typically comprises a moving pin associated with the cantilevered mirror, and a stationary hole associated with the support structure (e.g., the chassis). Since during dithering (rotating) operation, there is no lateral motion of the pin within the hole, the required clearance inside the stationary hole need only be sufficient to accommodate process and temperature variations.
While the pin-in-hole arrangement may protect the flexure from yielding during overflexure or buckling, its assembly is often difficult. Assembly can be made easier by increasing the diameter of the hole. However, a larger hole diameter affords less protection against higher shock levels when compared to the level of protection afforded by a smaller hole diameter. Furthermore, once the pin is properly positioned inside the hole, should the level of shock protection need to be changed, the hole diameter itself must be changed.
In an attempt to overcome some of the problems inherent with the pin-in-hole arrangement, dithering assemblies have been equipped with shock mounts. Mounting the flexure to the mounting member via shock mounts, as described in, for example, U.S. Application entitled "FLEXIBLE DITHER MOUNT WITH ROTATION," Svetal et al., filed Sep. 3, 1998 with Express Mail Label No. EM351172541US, hereby incorporated by reference, advantageously permits the diameter of the stationary hole to be larger than the diameter would be without the shock mounts. However, this design may increase manufacturing costs as well as the overall size of the scanning mechanism. Having recognized these conditions, an improved scanning system resistant to shock and vibration is desired.